1. Field of the Invention
The present invention relates to the preparation of fuel components. In particular the invention relates to a process for producing fuel components comprising dimers of light olefinic hydrocarbons.
2. Description of Related Art
High octane gasoline components can be produced from light olefinic hydrocarbons by dimerization process in which the olefins are dimerized in the presence of an acidic catalyst like an acidic ion exchange resin. Typical feedstocks are C4-C6 isoolefins which can form a tertiary carbocation and give a highly branched dimer which has a better octane number than the less branched molecules. Typical examples of such isoolefins are isobutene and isoamylenes.
In order to produce selectively dimers and to achieve long catalyst life it may become necessary to mix to the feed with sufficient amounts of suitable polar component(s). Examples of such components include water, methanol, tertiary butyl alcohol (TBA), MTBE and similar oxygenates. According to a particularly interesting process, C4- and C5-olefins are dimerized in the presence of alcohol or another oxygenate in a reaction sequence comprising at least one distillation zone and at least one reaction zone. The reaction is carried out at conditions in which at least part of the olefins dimerize. The distillation zone is arranged after the reaction zone, and a flow comprising an oxygenate, such as alcohol or water or the product(s) of reaction(s) between alcohol or water and the olefin(s) present in the feed, or a mixture of any or all of these is circulated from the distillation zone back to the dimerization. At least one circulation flow is withdrawn from the side of at least one distillation column. The molar ratio of alcohol or other oxygenate and isobutene is adjusted to be small during the reaction, thus maintaining the rate of dimerization high.
We have found that, to some extent, the polar component also reduces catalyst activity. This effect is not too significant when the concentration of the component to be dimerized is high, but if very high conversion is required, and the whole conversion is made in a simple once-through mode, the reaction rate can be very low near the outlet.
There are some possibilities of solving the problem. One option is to split up the reactor train in two parts and having a separation stage introduced between the reactors so as to reduce oxygenate level of the reaction mixture before it is fed into the secondary reactor train. A process configuration of this kind is depicted in FIG. 1 and it solves the reaction rate problem at the cost of lower selectivity and reduced catalyst life time in the secondary reactor train.
An alternative solution would be to replace the secondary reactor(s) of the previous embodiment with equipment in which the remaining reactants can be recovered as such and returned back to the primary reactors instead of forcing the dimerization reaction to completion by very severe reaction conditions. FIG. 2 depicts the block scheme of such a plant.
The arrangement of FIG. 2 has many advantages over the arrangement of FIG. 1, such as better product quality and longer catalyst life because the secondary stage is eliminated. However, a problem remaining with the embodiments of FIG. 2 is how to separate the reactants from the remaining inert hydrocarbons. The traditional method of separating components from hydrocarbon mixture is distillation. Technically distillation can be applied here, too, but because usually inert components and the reactants have boiling points very near to each other (e.g., feed contains mostly isobutane and isobutene), the column would be a typical superfractionator with very large dimensions and high energy consumption.
It is an aim of the present invention to eliminate the problems of the prior art and to provide a novel process for separating and circulating unreacted reactants during the production of fuel components based on dimerization of light olefinic feedstocks.
The present invention is based on the idea of recovering the unreacted reactant from the reactor effluent by using a reversible chemical reaction, in which the reactant is first converted to an intermediate product which can easily be separated from the mixture and this intermediate product is then decomposed back to the reactant and returned to the primary reactor feed.
More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.
Considerable advantages are obtained by the invention. Thus, by combining the dimerization process with chemical recovery of the light olefin reactant, the yield and catalyst lifetime problems of the direct dimerization process can be solved.